A load transmission device is provided with a first load-receiving member and a second load-receiving member that is assembled so as to be relatively movable with respect to the first load-receiving member, and a pressure chamber is formed as a compartment between these load-receiving members. The pressure chamber is filled with a pressure-transmitting medium, and a pressure, that is, a load applied by the load-applying means is transmitted to the first and second load-receiving members through the pressure-transmitting medium. With respect to the pressure-transmitting medium for use in such a load transmission device, a fluid such as hydraulic oil and nitrogen gas is generally used.
With respect to devices using hydraulic oil as the pressure-transmitting medium, a hydraulic nut and a bolt tensioner have been known, which preliminarily apply a load in a tensile direction to a bolt when fastening a member to be fastened by using a nut and the bolt.
The hydraulic nut is provided with a first ring-shaped member in which a screw hole and a cylinder through which a bolt is engaged with a screw are formed and a second ring-shaped member in which a fastening face that is made in contact with the member to be fastened and a piston that is attached so as to be movable in the axis direction relative to the cylinder are installed, and a pressure chamber is formed as a compartment between the cylinder and the piston. Further, a hydraulic pressure, that is, hydraulic oil is supplied from a hydraulic pump to the pressure chamber so that the first ring-shaped member is moved in the axis direction with respect to the second ring-shaped member, thereby making it possible to apply a load in the tensile direction onto the bolt.
Meanwhile, the bolt tensioner is provided with a housing that is placed to the member to be fastened, a load-receiving piston that is attached to the inside of this housing so as to be relatively movable in the axis direction, and a load-receiving cover that is secured to the housing to form a pressure chamber together with the load-receiving piston. Then, by supplying hydraulic pressure to the pressure chamber from the hydraulic pump, it is possible to apply a load in the tensile direction to the bolt through a rod that is secured to the load-receiving piston.
Moreover, with respect to the load transmission devices, those using gas such as nitrogen gas as the pressure-transmitting medium have been known, and as the load transmission devices of this type, an accumulator, which is installed in a hydraulic pipe connecting the hydraulic pump and a hydraulic apparatus, has been known.
The accumulator is used for effectively utilizing energy as an auxiliary power source and obtaining a high flow rate that greatly exceeds the pump capability by accumulating and discharging hydraulic pressure generated by a hydraulic pump. In addition, it is used for absorbing a surge pressure that occurs due to pulse movements of the hydraulic pump and a sudden close of a valve of the pipe system. For example, in the case of an accumulator of a bladder type, a housing, which is made of metal with a hollow space therein and connected to the hydraulic pipe, and a partition wall made of rubber and referred to as a bladder, which is installed inside the housing and partitions the inside of the housing into a pressure chamber for sealing nitrogen gas therein and an oil chamber that communicates with the hydraulic pipe, are installed. Thus, it becomes possible to absorb pulse movements and a surge pressure that occur in the hydraulic oil due to compressing property of the nitrogen gas sealed in the pressure chamber.
In the load transmission devices of this type, since the pressure-transmitting medium is a fluid such as hydraulic oil and nitrogen gas, and since the pressure-transmitting medium to be supplied to the pressure chamber is subjected to a load to increase the pressure, seal members are placed in a sliding portion and the like such as a cylinder and a piston that form a pressure chamber. For example, in the hydraulic nut, the seal member is placed on the circumferential portion of the piston that slides on the cylinder, and in the accumulator, the seal member is placed on a lid member that closes a nitrogen gas inlet. With respect to these seal members, O-rings, U-packing, X-rings and the like have been used in many cases, and in these cases, elastic materials such as rubber and plastic are used as the materials thereof; thus, even in the case when a gap in the sliding parts is increased by an applied pressure distortion, the seal member is elastically deformed in accordance with a change in this gap to maintain the sealing property.